1. Field of the Invention
The present invention relates to a photomultiplier assembly for use in cameras for detecting gamma rays, high sensitivity photometering devices for metering feeble light, etc.
2. Related Background Art
Recently, in the field of nuclear medicine, a diagnostic method in which radioisotope is given to a patient to measure a distribution image of the radioisotope has been rapidly developed. In this diagnostic method, gamma cameras for detecting gamma rays emitted from the radioisotope are generally used to obtain distribution images of the radioisotope those cameras being referred to as gamma cameras.
FIG. 1 is a vertical sectional view schematically showing one example of the detection units of the conventional generally used gamma cameras, i.e., gamma camera heads as shown in "Journal of Japanese Association of Radiation Techniques", March, 1971, p. 40. This gamma camera head 1 includes a box-shaped lead shield or lead housing 2 with one side opened. In the lead housing 2, there is a photomultiplier assembly 3 comprising a plurality of photomultipliers 4 held in a predetermined two-dimensional array. A collimator 5 is disposed in the opening of the lead housing 2. This collimator 5 comprises a lead plate with a great number of pores formed in parallel therethrough. Between the collimator 5 and the photomultiplier assembly 3, there are provided a scintillator 6 of sodium iodide and a light guide 7.
In such a gamma camera head 1, gamma rays entering in substantially parallel with the pores of the collimator 5 are incident on the scintillator 6, and the scintillator 6 emits light. As seen in FIG. 2 enlarging a part Of the light guide 7 and the photomultiplier assembly 3 with the scintillator not shown, light (solid lines) from the scintillator 6 passes through the light guide 7 and enters the photocathode surfaces 8 of the respective photomultipliers 4 arranged in a honeycomb structure behind the light guide 7. When the light arrives at the photocathode surfaces 8, photoelectrons are emitted, and the photoelectrons are multiplied gradually by groups of dynodes (not shown) in the photomultipliers 4 by the secondary electron emitting effect. The multiplied electrons are taken out as output pulse signals from an anode (not shown).
In this case, the most proximate of the photomultipliers to incident positions of the gamma rays receives a largest quantity of light. The photomultipliers which are more distant from the incident positions of the gamma rays receive the smaller quantities of light. Quantities of light distributed to the respective photomultipliers 4 are proportional to solid angles defined by light emitting points and the photocathode surfaces of the respective photomultipliers 8 as to those of the photomultipliers near the light emitting points. When the respective photomultipliers 4 receive the light, they output pulse signals with wave crests proportional to their incident light quantities. Accordingly, output signals of the photomultipliers 4 are larger as the photomultipliers 4 are located closer to incident positions of the gamma rays, and smaller as the photomultipliers are located more distant from incident positions of the gamma rays.
Accordingly, large and small signals from the respective photomultipliers disposed in a certain array are compared and computed by a position computing matrix circuit 9 disposed in the lead housing 2 so that incident positions of gamma rays are indicated by X-Y coordinate signals. Based on this coordinate signals, luminescent points can be generated at positions corresponding to the incident positions of gamma rays on a screen of a display device (not shown). The luminescent points on the screen are imaged by an optical camera to be accumulated on a frame of film. Thus, a scintigram related to a distribution of radioisotope in a patient's body can be recorded.
Such a gamma camera head 1 is not only for giving the two-dimensional distribution image of radioisotope, but also is used in a single photon emission computer tomography (SPECT).
In the photomultiplier assembly 3 of the gamma camera head 1 described above, when the photocathode surfaces 8 of the respective photomultipliers 4 are circular, as shown in a transverse cross section of the gamma camera head 1 of FIG. 3, gaps or dead spaces 10a, 10b are defined respectively among adjacent three photomultipliers, and between the inside surface of the lead housing 2 and the peripheral photomultipliers. Light which has entered these dead spaces 10a, 10b is not used. Resultant problems are that the condensing efficiency is accordingly lowered, and a resolution of the gamma camera is lowered.
To solve these problems, conventionally as shown in FIGS. 1 and 2, triangular pyramidal cuts or recesses are formed in a light guide 7 at positions corresponding to the dead spaces 10a, 10b, and triangular pyramidal reflectors 11a, 11b are placed in the recesses. In this arrangement, as shown in FIG. 2, light entering the respective dead spaces 10a, 10b reflects on the surfaces of the reflectors 11a, 11b, a part of the light enters the photocathode surfaces 8.
However, in the gamma camera head 1 with the reflectors 11a, 11b disposed in the light guide 7, as seen in FIG. 2, the peripheral edges of the photocathode surfaces of the photomultipliers 4 partially overlap the bottoms of the reflectors 11a, 11b. Accordingly, the light entering from the forward (from above as viewed in FIG. 2) does not reach the peripheral portions of the photocathode surfaces 8. A resultant problem is that parts of the photocathode surfaces cannot be efficiently used. Further, in this arrangement, a most part of light entering the dead spaces 10a, 10b is reflected forward, and only a part of the light enters the photocathode surfaces 8. A resultant problem is that use efficiency of light is low.
Also, a method for eliminating dead spaces by means of using photomultipliers 4' whose photocathode surfaces 8' are hexagonal as in FIG. 4, or quadrangle although not shown is well known. This method can eliminate dead spaces among the photomultipliers 4'. However, it cost more to fabricate hexagonal or quadrangle photomultipliers 4' than to fabricate photomultipliers 4 with the circular photocathode surfaces 8. Further, even in the case that hexagonal photomultipliers 4' are used, dead spaces 10' among the lead housing 2' and the photomultipliers 4' cannot be eliminated.
Japanese Patent Laid-Open Publication No. 2-304316 (304316/1990) describes a high sensitivity photometering device including photomultipliers comprising a cylindrical transparent vessel having all the circumferential wall formed in a photocathode surface. This device is for efficiently collecting light from the sides of the photomultiplier. Accordingly, means for solving the problem of the dead spaces defined in the photomultiplier assembly is not described and taught in Japanese Patent Laid-Open Publication No. 2-304316.